Question sessions

The purpose of a research School is to provide young scientists the basics in a working field. With this respect interactivity between students and lecturers should be promoted. Thus a key aspect of ESM is the possibility to raise questions during the course of the lectures, interrupting the lecturer. Questions can also be posted anonymously in 'question boxes'. Special sessions for answering these questions are organized, typically 1h every day or two days, during which the lecturers or voluntary students present in more detail issues raised by the students during the lectures or anonymously through a question-box. Questions raised and explained in 2017 are being listed below on a regular basis. See also the questions raised in 2015, questions raised in 2013.[L. Ranno] What is the difference between N and D demagnetizing factors?

1. [L. Ranno] You said that susceptibility χ is a scalar. Is it always the case ?
2. [W. Wulhekel] When do you measure macroscopically m0=gJ and when meff=g * squareroot[J(J+1)]. What is the difference between effective magnetic moment µeff and total magnetic moment
3. [W. Wulfhekel] Can you clarify what is the anisotropy and the different type of it such as crystalline anisotropy, local anisotropy etc...?
4. [W. Wulfhekel] Why does an antiferromagnetic material have a magnetic moment ?
5. [O. Isnard] What is the difference between mumetal and permalloy ?
6. [O. Fruchart] What are Néel and Bloch walls? How is it conected with the Dzyaloshinskii-Moriya DM interaction? Can you say something about dynamics DM interaction?
7. [O. Fruchart] Can magnetic field microscopy be used while applying significant fields? Are hysteresis measurements possible?
8. [O. Fruchart] What are Néel and Bloch walls? How is it connected with Dzyaloshinskii-Moriya interaction? Can you say something about dynamic Dzyaloshinskii-Moriya interaction?
9. [C. Back] As the different temperature transitions (Tb, Tc ...) are dependant from the dynamic measurement, is there any rule, to compare these temperatures from different apparatus type of measurement? Can you give the time measurement of SQUID, XMCD, Mössbauer, where can we find this?
10. [O. Fruchart] In Foucault mode image, we block some position of the incoming beam. Do we block by some aperture? This mode helps us to image single domains. I also listen that by dark/bright field imaging (by blocking/opening direct/indirect beam of difraction spot) hemps us to identify singel domain. Can you comment on this please?
11. [L. Ranno] You told XMLD help us to probe antiferromagnetism. Can you explain how? There is a highly debated topic how to see antiferromagnetic skyrmions. Do you think XMLD help us to image antiferromagnetic skyrmions? Please comment on it.
12. [O. Fruchart] STM imaging is based ontunneling phenomena. Can you comment about the contrast of the imaging? What are the advantages and disadvantages of scanning tunneling spectroscopy microscopy versus regular STM (on a comparison basis).
13. [I. Mertig] In the Heisenberg model with more complex interactions, one writes the exchange inetraction as a tensor. Some describe it as partly an Ising interaction, writing it as H=Σ S_i.I_ijS_j, I_ij being a tensor. How can one understand it in the context of the Ising model?
14. [I. Mertig] What is the difference between itinerant and indirect exchange? Both include itinerant electrons.
15. Schottky anomaly observed by heat capacity measurements with dependence on temperature can give the value of the spin gap, is it?
16. [C. Back] Could someone explain the difference, if there is, between spin waves and magnons?
17. [I. Mertig] How the lack of symmetry at the interface between a ferromagnetic material and a non-magnetic material may give rise to a Dzyaloshinskii-Moriya Interaction (DMI).
18. How can the Dzyaloshinskii-Moriya Interaction (DMI) be measured? Is the DMI higher, lower or equal if the magnetization is in-plane or out-of-plane?
19. [L. Chapon] How is the modulation of the magnetic structure of a magnetic crystal that has monoclinic symmetry at low temperature (please derive step by step)
20. [I. Mertig] How to observe the localisation / delocalisation of magnons experimentally? What is "magnon vacuum" (experimentally)?
21. Is there any nomenclature for magnetic irreps? For instance when two have k vetcor on the Γ point of the Brillouin zone, I have to write Γ_mag=1Γ1 + 1Γ2 + 2Γ3,while if I have k on the lambda point of the Brillouin zone, (here lambda locked to (001)). I write Γ_mag= 1GΛ1 + 1Λ2 + 2Λ3. Can we simply use thany symbol to represent the irreps?
22. What is the physical meaning of complex basis vectors ?
23. The Landau Theory states that a 2nd-order transition can involve the build-up of magnetic fluctuations that have the symmetry of only one irrep, what happens if we have a first order transition? Are more irreps involved?
24. Is it possible to have a secondary order parameter (which cannot be described with the irrep corresponding to the primary order parameter) ?
25. By symmetry arguments we can determine whether α_E, α_EE or EE (etc) effect is supported by a certain material or not. Is this true for any type of system? What about multilayer structures such as ferromagnetic Co/Pt ?
26. What is a group structure ?
27. Is symmetry breaking necessary for 2nd order transition ?
28. Why is the ratio g/h between the order of group and subgroup an interger?
29. Is conductivity of d-electrons always worse than for s and why ?
30. What is diffraction versus scattering ? Difference? Definition? Examples?
31. Can you comment, for an antiferromagnet : Tc versus θ. What is the difference?
32. [L. Ranno] Do electrons feel induction B or magnetic field H?
33. [L. Chapon] What is ferroelasticity? Give examples.
34. [I. Mertig] Why is ε(k) called a dispersion relation?
35. [I. Mertig] Is there any way to derive the Pauli principle?
36. [L. Chapon] Why is Q x m x Q a projection?
37. [I. Mertig] What is the difference between superexchange and double exchange?
38. [I. Mertig] What does bosonisation mean?

39. What is spin momentum locking ?

40. What are the differences and the similarities between free space electron momentum and crystal (quasi) momentum ?

41. Could you please explain spin pumping and spin mixing conductance ?

42. L. Ranno showed that we can use exchange-bias for magnetoresistance. He showed the positive exchange-bias coupling. I heard there is also a negative exchange-bias FM -> -> AFM <- -> <- -> (example sketch of negative EB). Can someone comment about this and explain where this originates from and why ?

43. About the microscopic origin of the interlayer exchange coupling beyond RKKY. How can interlayer exchange coupling be described as a quantum size effect?

44. What is the physical origin of TAMR (Tunneling Anisotropic Magnetoresistance). What is the difference between TAMR and ATMR ?
45. [C. Back] Is spin-orbit required for anisotropy?
46. [V Simonet] Do you need to break both inversion and time-reversal symmetry for DM interactions?*
47. [B Canals] What is the origin of RKKY interactions?
48. [V SImonet] It has been mentioned that neutron nuclear scattering would never change direction of polarization. Is that tru when structure is chiral (structural chirality connected with lattice, not magnetic chirality)
49. [H. Ronnow] What does localized electrons versus delocolized electrons look like?
50. [A. Cano] What is the difference between DFT and second quantization? Both create an effective picture of the many-body system. Can you do ab-initio calculations in second quantization, eg tight binding?
51. Can you please develop: the mass-relativity term Hmv? The term H_D
52. [B Canals] Can you give a practical example of a physical thing which is the Berry phase, the Berry curvature and a Berry field,
53. [O Fruchart] Does the current injected from an STM tip into a sample change the electronic structure of the sample? Can influence the sample in any way?
54. [M Kenzelmann] Could you please comment on "hidden order".
55. [M Kenzelmann] I was confused about the positions with spins labeled "0" in the cartoon picture. It is related to spin waves?
56. [B Canals] Could you explain the difference between interactions and correlations?
57. [ O Fruchart] How can one observe experimentally embedded (in bulk) Bloch points?
58. [B Canals] Can you explain (again) deconfinement / confinement with respect to defects and the arrow of time? In 1D, 2D, 3D.
59. [V Simonet] Is it necessary to consider a material without inversion center to get DM interaction?
60. [H. Ronnow] Can you please explain about the different spinwave modes such as i) propagating modes (MSSW and MSBVW) ii) Localized modes, spinwave bullet, droplets, etc.?
61. [H. Ronnow] In 2D topological insulators, we have non-dissipative currents at the edges of the structure (with different directions and spins of e). How can we experimentally measure the absence of resistance of the currents (to non-magnetic impurities) if the total current in one edge is equal to zero?
62. [H. Ronnow] What is the difference between spinwave, magnon, and spinons? Why spinwaves cannot appear in 1D?
63. [O. Fruchart] Can you comment on changes of magnetic anisotropy in thin films? Is perpendicular anisotropy always favourable? On what it depends?
64. [V. Simonet] Could you explain "step-by-step" thermal dependence of magnetization of classic antiferromagnet in ZFC FC regimes for different orientations of external magnetic field? Is it possible to obtain hysteresis loop in M(H) dependence in antiferromagnets?
65. [V. Simonet] In SQUID measurements of temperature evolution of magnetic susceptibility, there are two modes to choose from: the field-cooling (FC) mode and the zero-field-cooling (ZFC) mode. Usually, both modes of measurements are performed. What information can we obtain from both measurements?
66. [O. Fruchart] Easy question: Considering a permanent magnet with a cycle of hysteresis. The working point is the point where (BH)max is optimized due to the aspect ratio (AR) of the permanent magnet. Question is : What is the shape of the hysteresis loop if I put a sample of this AR into the VSM (coercive field the same)? How to correct it to have M(Hint) instead of Hext?
67. [H. Ronnow] What does localized electrons versus delocalized electrons look like?
68. [A. Cano] Could you please comment on "hidden order".
69. [O. Fruchart] How can one observe experimentally embedded (in bulk) Bloch points?